A conventional distributor for an internal combustion engine is shown in FIG. 1. A distributor housing 10 mounted in an internal combustion engine (not shown) consists of two portions, namely an upper distributor housing portion 11 and a lower shaft bearing portion 12. The lower portion 12 in a cylindrical shape has an axial bore 13 in which a rotary shaft 14 is inserted and supported at the upper end by a bearing member 15 and at the lower end by a metal sleeve 16. The rotary shaft 14 is at the upper end integrally connected with a bushing 17 upwardly extending axially of the enlarged space 18 in the upper portion 11 of the housing 10. The upper portion 11 is at its upper end engaged with and closed by a cap 19 secured to the housing 10 by screws 20. The bushing 17 has a rotary disc 21 secured between upper and lower parts 17a, 17b vertically divided and fixed by a screw 22. A rotary disc 21 extends radially and outwardly of the bushing 17. The bushing 17 at its top end is connected with a distributor rotor 23 which is secured by inserting the upper end of the bushing 17 into a bore provided in the distributor rotor 23 and fixed by a screw 24.
The distributor rotor 23 is made of synthetic resin and at its upper surface has a rotor electrode 25. The rotor electrode 25 is at its one end positioned about the rotating axis of the rotor 23 and is slidably connected with a contact 26 provided at the lower end of a central electrode 27, which is arranged in alignment with the rotary axis of the distributor rotor 23 and secured in a bore provided in the top wall of the cap 19. The electrode 25 at its other and outer end moves and passes near the side wall of the cap 19 and, successively, a plurality of circumferential electrodes 28 with a predetermined discharge gap therebetween. The circumferential electrodes 28 are positioned about the central electrode 27 circumferentially of the top wall of the cap 19 and spaced one to the other with a predetermined angular interval. A metal plate 39, best seen in FIG. 1, rotates with the rotor and generates air circulation in the rotor space 18.
The shaft 14 is rotated synchronously with the revolution of the internal combustion engine and rotates the disc 21 integrally therewith. The disc 21 has a radial slit (not shown) located at a position at which it faces to a photoelectric pickup. The photoelectric pickup consists of a light-emitting element 29 and a light-receiving element 30 which are arranged opposite to each other with respect to a symmetric plane containing the rotary disc 21. The unit 31 supported on a stepped and annular shoulder 11a, including the light-emitting and light-receiving elements 29, 30 also has a signal processing circuit 32 for processing output signal from the photoelectric pickup. In the above-mentioned apparatus, the shaft 14 rotates synchronously with the revolution of the engine (not shown). During rotation of the shaft 14 the slit in the rotary disc 21 causes infrared light beam intermittently passing it between the light-emitting element 29 and the light-receiving element 30. The output signal from the light-receiving element 30 is processed for the wave-form shaping and the like by signal processing circuit 32 thereby to obtain a predetermined crank angle signal or cylinder discriminating reference signal. Such a signal determines an ignition time when the ignition high voltage is supplied to the central electrode 27, where it is successively distributed to a plurality of circumferential electrodes 28 through the contact 26 and the rotor electrode 25 so that ignition is provided to the full charge in each cylinder of the internal combustion engine (not shown).
As shown in FIG. 2, the distributor rotor 23 of cylindrical shape at its upper surface has the rotor electrode 25 extending from the central portion laterally or radially. The rotor electrode 25 is generally in the shape of the letter T, a vertical portion 33 of which is fixed to the distributor rotor 23 and a horizontal portion 34 of which successively faces circumferential electrodes 28 when rotated. The surface 35 of the horizontal portion 34 opposing to the circumferential electrode 28 has a shape of circular section or segment and this segment portion is coated with dielectric material 135, such as silicon varnish, for suppressing RFI.
Such conventional apparatus as stated above, however, has gaps in the distributing line and as discharge from the rotor electrode 25 is initiated at a high voltage (for example 10-15 KV) to the circumferential electrode 28, high frequency (for example 500 MHz to 1 GHz) in this time produces the RFI causing difficulties to raise noise voltage in an electronic circuit such as the signal processing circuit which voltage causes operational errors. It has been known that in order to solve the problem a discharge starting voltage is suppressed by applying grease and the like to the end surface of the rotor electrode. However, even in such an improved apparatus the grease is carbonized by continuous discharge and, in a relative short time, the beneficial effect of grease for suppressing discharge starting voltage is considerably reduced and, accordingly, the RFI increases.
Various other proposals have been made for suppressing RFI by coating the surface of rotor electrode opposing the circumferential electrodes by a dielectric material such as silicon or the like. U.S. Pat. No. 4,074,090 discloses silicon coating disposed on the surface of the discharge electrode. U.S. Pat. No. 4,631,369 discloses wire net embedded in dielectric material such as silicon varnish coating layer coated on the discharge electrode surface.
However, in the presence of the silicon dielectric material fully coated on the discharging end of the rotor electrode, intermittent discharges arise due to charged particles accumulated in the dielectric material during the induced discharge, whereby a sufficient effect for suppressing the RFI is not attained. Furthermore, being exposed to the discharge, the silicon dielectric material is easily exfoliated and drops from the end of the rotor electrode so that it can not retain the RFI suppressing effect for long periods of time, resulting in poor durability and operability. On the other hand, since the dielectric material is coated onto the top end of the rotor electrodes one at a time, the dielectric material is not practical from the view of the mass-production thereof.